Precision Neutron Decay Measurements

Tulane University, New Orleans LA

Investigators

Abstract

The neutron is a key building block of ordinary matter. More than half of the Earth's mass is from neutrons, but when freed from the confines of an atom, the neutron is unstable and decays into other elementary particles: a proton, an electron, and an antineutrino (a lightweight neutral particle), with an average lifetime of about 15 minutes. Neutron decay is a useful laboratory for studying details of the force responsible - the weak nuclear force - one of the four fundamental forces of nature. This award will support experiments using beams of free neutrons passing through specialized detectors that will precisely measure the neutron decay lifetime and the angles between the emitted particles. These results test and lead to refinements of the weak interaction theory and a better understanding of the physics of the sun, stars, the Big Bang, and important nuclear reactions. This work, at the "precision frontier" of particle and nuclear physics, complements research at the "high energy frontier", for example at the Large Hadron Collider in Europe. This program includes excellent opportunities to train undergraduate and graduate students in the general methods and theory of neutron science that are applicable to diverse fields in physics, chemistry, materials science, and biology research at major neutron sources around the world. The beta decay of the free neutron is the prototype semileptonic weak interaction and simplest nuclear beta decay. There are no complications from nuclear structure, and the decay energy is small compared to the nucleon mass, so recoil-order weak form factors enter below the 0.1% level. Therefore, neutron decay is an attractive system for precise low energy weak interaction measurements. The neutron lifetime establishes the time scale and temperature of nucleon "freeze out" shortly after the Big Bang, which sets the neutron to proton ratio during the era of primordial nucleosynthesis and thence the helium abundance, and indirectly constrains the effective number of light neutrinos. The electron-antineutrino correlation a-coefficient gives lambda, the ratio of axial vector (GA) and vector (GV) weak nucleon couplings. With the neutron lifetime it can be used to determine Vud, the first element of the Cabbibo-Kobayashi-Maskawa matrix, and constrain new physics beyond the Standard Model such as weak scalar and tensor forces. There is a serious problem with recent neutron lifetime experiments: results obtained using the traditional beam method, and those using the newer ultracold neutron storage method, disagree by about 1% (4 standard deviations). An important goal of this research program is to resolve this troubling discrepancy. The program consists of two main projects: 1) completing the BL2 neutron lifetime experiment now running at the NIST Center for Neutron Research; 2) development and operation of the new BL3 next-generation beam neutron lifetime experiment that will investigate and test systematic effects that can help resolve the neutron lifetime discrepancy and improve the precision of the beam method to less than 0.04%. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.